In an “E-UTRAN (Evolved-Universal Terrestrial Radio Access Network)/EPC (Evolved Packet Core)” mobile communication system defined by 3GPP, a mobile station UE is configured to connect with a “Default APN (Access Point Name)” specified by a network during an Attach process.
To be more specific, as shown in FIG. 5, after initiating the Attach process within an area in the E-UTRAN, the mobile station UE transmits an “Attach request (Attach request)” to an E-UTRAN packet exchange MME in Step S100.
In Step S101, the exchange MME downloads subscriber data (including Default APN information) on the mobile station UE from a subscriber accommodation station.
In Step S102, the mobile station UE executes a connection process with the “Default APN (e.g., IMS (IP Multimedia Subsystem))” in response to an instruction from the exchange MME.
In Step S103, a bearer is established between the mobile station UE and the “Default APN.”
The mobile station UE is configured to execute a location registration process to the E-UTRAN or UTRAN, when moving between an area in the E-UTRAN and an area in UTRAN (Universal Terrestrial Radio Access Network).
To be more specific, as shown in FIG. 6, in Step S111, the mobile station UE in a UTRAN location registration area RA#1 (state (A)) transmits an “Attach request (RA#1)” to a UTRAN packet exchange SGSN.
The UTRAN packet exchange SGSN instructs a subscriber database HSS (Home Subscriber Server) to update location registration information on the mobile station UE in Step S112, and then transmits an “Attach accept (including information on RA#1)” to the mobile station UE in Step S113.
Here, a “GMM context” that is a UE context of the mobile station UE in the UTRAN is retained by both the mobile station UE and the UTRAN packet exchange SGSN.
Upon detection of its movement from the UTRAN location registration area RA#1 to an E-UTRAN location registration area TA#1 (state (B)), the mobile station UE transmits a location registration request (TA#1) to the E-UTRAN to the E-UTRAN packet exchange MME in Step S121.
The E-UTRAN packet exchange MME instructs the subscriber database HSS to update the location registration information on the mobile station UE in Step S122, and then transmits a “location registration response (including a TA list with information on TA#1 and TA#2)” to the mobile station UE in Step S123.
Here, in the mobile station UE, the “GMM context” described above is deleted, and an “EMM context” that is a UE context of the mobile station UE in the E-UTRAN is retained instead. Note that the “EMM context” is also retained by the E-UTRAN packet exchange MME.
Thereafter, upon detection of its movement from the E-UTRAN location registration area TA#1 to an UTRAN location registration area RA#1 (state (C)), the mobile station UE transmits a location registration request (RA#1) to the UTRAN to the UTRAN packet exchange SGSN in Step S131.
In addition, upon detection of its movement from the UTRAN location registration area RA#1 to an E-UTRAN location registration area TA#2 (state (D)), the mobile station UE transmits a location registration request (TA#2) to the E-UTRAN to the E-UTRAN packet exchange MME in Step S141.
Generally, as shown in FIG. 7, the mobile communication system described above is configured so that when the mobile station UE is in the E-UTRAN location registration area, the connection between the mobile station UE and the “Default APN” (more specifically, a connection for the mobile station UE between the “Default APN” and a “Serving-GW”) is maintained even if an RRC connection with a radio base station eNB is shifted to an Idle state (non-communication state).
However, the number of foreign networks (APNs) that one mobile station UE can access simultaneously in the UTRAN is smaller than that in the E-UTRAN. This leads to a case where the connection between the mobile station UE and the “Default APN” is cut off while the mobile station UE is within the area in the UTRAN.
As a measure against such a case, 3GPP defines a method for encouraging the mobile station UE to execute a connection process with the “Default APN” when the mobile station UE moves back to the area in the E-UTRAN from the area in the UTRAN.
To be more specific, as shown in FIG. 8, when the mobile station UE moves back to the area in the E-UTRAN from the area in the UTRAN, the mobile station UE transmits a location registration request to the E-UTRAN packet exchange MME in Step S150, and receives a location registration rejection response from the E-UTRAN packet exchange MME in Step S151. Specifically, the location registration rejection response instructs the mobile station UE to re-execute the Attach process.
In Step S152, the mobile station UE transmits an “Attach request” to the E-UTRAN packet exchange MME.
In Step S153, the exchange MME downloads subscriber data (including Default APN information) on the mobile station UE from a subscriber accommodation station.
In Step S154, the mobile station UE executes a connection process with the “Default APN (e.g., IMS (IP Multimedia Subsystem))” in response to an instruction from the exchange MME.
In Step S155, a bearer is established between the mobile station UE and the “Default APN.”
For the mobile communication system described above, a technique called “ISR (Idle Mode Signaling Reduction)” is defined.
In the mobile communication system using “ISR,” the mobile station UE does not have to execute a location registration process even when moving between the area in the UTRAN and the area in the E-UTRAN, thus enabling reduction in signals required for the location registration process.
To be more specific, as shown in FIG. 9, in Step S201, the mobile station UE in a UTRAN location registration area RA#1 (state (A)) transmits an “Attach request (RA#1)” to a UTRAN packet exchange SGSN.
The UTRAN packet exchange SGSN instructs a subscriber database HSS to update location registration information on the mobile station UE in Step S202, and then transmits an “Attach accept (including information on RA#1 and information indicating use of ISR)” to the mobile station UE in Step S203.
Here, a “GMM context” that is a UE context of the mobile station UE in the UTRAN is retained by both the mobile station UE and the UTRAN packet exchange SGSN.
Upon detection of its movement from the UTRAN location registration area RA#1 to an E-UTRAN location registration area TA#1 (state (B)), the mobile station UE transmits a location registration request (TA#1) to the E-UTRAN to the E-UTRAN packet exchange MME in Step S211.
The E-UTRAN packet exchange MME instructs the subscriber database HSS to update the location registration information on the mobile station UE in Step S212, and then transmits a “location registration response (including a TA list with information on TA#1 and TA#2)” to the mobile station UE in Step S213.
Here, the mobile station UE does not delete the “GMM context” described above, but retains it together with the “EMM context.” Note that the “EMM context” is also retained by the E-UTRAN packet exchange MME.
Thereafter, even when detecting its movement from the E-UTRAN location registration area TA#1 to the UTRAN location registration area RA#1 (state (C)), the mobile station UE does not transmit a location registration request (RA#1) to the UTRAN to the UTRAN packet exchange SGSN since it retains the “GMM context.”
Also when detecting its movement from the UTRAN location registration area RA#1 to the E-UTRAN location registration area TA#2 (state (D)), the mobile station UE does not transmit a location registration request (TA#2) to the E-UTRAN to the E-UTRAN packet exchange MME since the movement is between the TAs in the TA list and the mobile station UE retains the “EMM context.”